May want to explore dark0's angle then. If your card was overclocked out of the box (compared to reference specs), something may be degraded just enough now to put it on the edge of stability now. Cooling may not be as efficient, PSU may be on the lower side of efficiency, or just wear and tear from stress over time, etc. May be a good idea to check for the dust bunnies... Do a little belated spring cleaning so to speak. Might even need to do a little downclocking from the stock settings.